texts: Do we still use the term "astronomical unit" nowadays?

texts: An astronomical unit is defined as the measurement of distance between Earth and our Sun, my question is since distances between celestial objects beyond our solar system are vast and unimaginable astronomer adopted another familiar term "light-year" instead. My question is do people still use astronomical unit in the present days?


texts: What is happening with these solar particles detected near the Sun that is so newsworthy?

texts:
Phys.org's Team identifies low-energy solar particles from beyond Earth near the Sun links to the open access ApJ article Properties of Suprathermal-through-energetic He Ions Associated with Stream Interaction Regions Observed over the Parker Solar Probe's First Two Orbits.

It quotes "Dr. Mihir Desai, a mission co-investigator on the Integrated Science Investigation of the Sun (IS☉IS) instrument suite, a multi-institutional project led by Principal Investigator Prof. Dave McComas of Princeton University.. IS☉IS consists of two instruments, Energetic Particle Instrument-High (EPI-Hi) and Energetic Particle Instrument-Low (EPI-Lo)" as saying:

  With EPI-Lo, we were able to measure extremely low-energy particles unexpectedly close to the solar environment. We considered many explanations for their presence, but ultimately determined they are the smoking gun pointing to interactions between slow- and fast-moving regions of the solar wind that accelerate high-energy particles from beyond the orbit of Earth. Some of those travel back toward the Sun, slowing against the tide of the outpouring solar wind but still retaining surprisingly high energies.

I'm having trouble parsing this very long single sentence. Why are these "extremely low-energy particles" also "still retaining surprisingly high energies"?

The Phys.org article talks about Stream Interaction Regions (SIRs) and the formation of turbulent corotating interaction regions (CIRs) that produce shock waves and accelerated particles, not unlike those produced by CMEs.

Question: I think it is saying that particles from the Sun go beyond Earth and then are turned around by turbulence and move back into close proximity of the Sun, at which time they have "extremely low-energy" for some reason. Is that at least a rough understanding of what's happening?


texts: Do other planets with a magnetic field also have two Van Allen belts?

texts:
On Jupiter and Saturn, aurora is seen around the planet's poles, so that implies that there must be a kind of magnetic field around those planets. Perhaps even the moon Europa could have a magnetic field created by salty water (Does it?).

On Earth we have two radiation belts, an inner and outer belt. One contains electrons and the other mostly protons. Do other planets also have these two radiation belts?


texts: Can black holes be used as an energy source

texts: I’ve heard that black hole will be rotating continuously which stores a huge amount of energy in its rotation. Will this energy be accessible, since the rotation is imposed on the space outside the hole.


texts: What kind of triangle is formed by three unequal masses in a circular restricted three body orbit?

texts:
This answer to Are the orbits of all triple star systems at least technically unstable? mentions:

There are known solutions to the gravitational three body problem that can be shown to be stable. Lagrange found a three body solution for general masses where all three orbit the common center of mass in an equilateral triangular formation. Gascheau proved in 1843 that this solution is stable if the component masses satisfy
$$ \frac{m_1 m_2+ m_1 m_3 + m_2 m_3}{(m_1+m_2+m_3)^2} < 1/27$$

When the smallest mass approaches zero the three masses are at the vertices of an equilateral triangle. In a realistic solar system this means Trojan asteroids are generally found in orbits of massive planets like Jupiter about 60 degrees ahead-of and behind it.
But if the smallest mass is large but the inequality above is still satisfied, what can we say about the triangle formed by the three bodies in a circular restricted three-body problem orbit?
Is it still known to be an equilateral triangle, but they spin around a point which is not the center of the triangle, but is weighted toward the heavier object?

If so, can this be shown by citing a math-based reference or shown here mathematically or computationally?
If not, is there an expression for two angles of the triangle as a function of to mass ratios?

 Source click for full size


texts: Brightness, altitude & azimuth for satellites?

texts:
I'm using the heavens-above website to identify a satellite I've seen several nights this week.

The question is related to how I can search the satellites table with the information I can provide.

The page has a table that has the satellite name, Brightness (mag), and Time, Altitude, and Azimuth for Start , Highest point and End. I don't know how brightness and altitude are measured, and I don't know how the start, highest, and end points are measured. In terms of azimuth, if I base it on this compass, and based on the time I saw it, I would have to say NNE.

With that said, last night I was sitting looking straight north. I first saw the satellite, literally moving directly over my head, at 7:33pm EST and stopped seeing it at a 7:36PM EST because it was too far. The satellite was going North and just a bit to the East.

My question: how can I use this information to search for the satellite in the table? Also, how do I know what's the Brightness (mag)? The satellite is so tiny that it's almost impossible to see with the naked eye.


texts: Does anything orbit the Sun faster than Mercury?

texts:
Mercury's orbital period around the Sun is about 88 days.  Comets and other things have gotten closer to the Sun than Mercury does.  But has there ever been an asteroid or some other body discovered that has a shorter orbital period?  Are there theoretical constraints on if such a body can exist?
The IAU defines a planet as one that has swept out its orbit.  Does the neighborhood of Mercury extend to the Sun?


texts:
This question could be strictly linked to my another question related to black holes.

It’s known that the  horizon event is a place where dust,star components are ready to be incorporated to the black hole*.

Actually, my question can be divided in few points:

Might be reasonable to believe that ,if it’s true that in many spiral galaxies there is a black hole in the middle, could these objects have so a powerful gravitational force to consider them the creators of the galaxies?
(*) when the black holes are sucking up dust from the horizon event, are they increasing their mass?

If so, it’s reasonable to believe that their gravitational force will increase in a way to attract farther objects?
Could they eject the dust or anything else if they are “eating” so much?
     This can be in contradiction with above point.


texts: Could neutrinos be at least a part of Dark Matter?

texts:
What we know about neutrinos - its are almost invisible the same as Dark Matter, right?
Why Dark Matter could not consist of neutrinos?


texts: How much (more or less) of the moon will be visible as it recedes from the Earth?

texts:
The Moon is receding from the Earth, at a rate of about 4 cm per year. We can currently see about 59% of the Moon's surface, from the Earth's surface.
Will the amount of the Moon's surface we can see from Earth change (significantly) as the Moon recedes?


texts:
The question Can we see asteroid 1998 OR2 with unaided eye? got me thinking. Space.com's Vesta: Facts About the Brightest Asteroid says:

  Vesta is the second most massive body in the asteroid belt, surpassed only by Ceres, which is classified as a dwarf planet. The brightest asteroid in the sky, Vesta is occasionally visible from Earth with the naked eye. It is the first of the four largest asteroids (Ceres, Vesta, Pallas and Hygiea) to be visited by a spacecraft. The Dawn mission orbited Vesta in 2011, providing new insights into this rocky world.

The brightness of an object seen from Earth is proportional to (among other things) $1/r^2$ so an object which occasionally passes very close to Earth will occasionally be far brighter than normal; an object normally of order 200 million km away that comes to within 200 thousand km will be a million times brighter, and at 2.5 magnitudes per power of ten that means that for a brief time it could be 15 magnitudes brighter than average.

Questions:

Has a near earth object in heliocentric orbit ever been bright enough to be visible to the unaided eye?
Are there any predictions of events in the foreseeable future when this will happen?

Please exclude comets which are themselves invisible and it's the giant clouds of dust and gas they produce that we see and planets (and dwarf planets) who's heliocentric orbits make them regularly visible.

The following may provide some helpful definitions:

Is there a distinction between NEOs and near-Earth asteroids? Is there a difference?
What (actually) defines an Aten-class near-earth asteroid?
Has Hubble ever been used to try to image a near Earth asteroid?
Is the passage of three asteroids near Earth today just coincidental?
https://space.meta.stackexchange.com/q/1459/12102


texts:
While messing around in https://dan-ball.jp/en/javagame/planet/. I came across a pretty stable orbit of 2 bodies around a star

Orbit yellow:

Orbit Blue:

full album: https://imgur.com/a/ruyenmY

Is this a know orbit type? if so what is the maximum distance between the inner circular orbit and outer circular orbit?


texts: When was the atmosphere of Venus first observed?

texts:
Nowadays, atmospheres of transiting exoplanets are being characterized. We can measure the absorption of the star's light through the exosphere of exoplanets.

The same can be done closer to Earth, with the inner planets. In fact, this was done during the 2012 Venus transit.

When is the first time that a transit was used to study the atmosphere of Venus ?


texts:
Something I've been wondering lately is how much the grand tack hypothesis stands up to present scrutiny. While the grand tack proposes that Jupiter and Saturn were caught in a 3:2 mean-motion resonance which results in migration outwards to roughly close to their present distances, simulations of dynamics of planetary migration seem to indicate that a 1:2 MMR is much more likely to occur, which most likely does not result in migration outwards.
Also, a much more recent hypothesis suggests that Jupiter formed much further out than the present location and then migrated in over a period of ~700,000 years, which would explain the distribution of Trojans in Jupiter's orbit. The starting scenario of this hypothesis is completely different from the grand tack which states that Jupiter formed around 3.5 AU distance. It also says absolutely nothing about Jupiter and Saturn migrating further inwards and then back out due to resonance.
Can the grand tack hypothesis be reconciled in any way from these newer studies?


texts: Is there a strong galactic magnetic field?

texts:
My main question is: Is there a strong galactic magnetic field, perhaps driven by the supermassive black hole at the center of our galaxy?  I am also wondering if this field would be strong enough to make it so that the galaxy rotates in the way it does (with the outer stars moving faster than would be expected), and if this would be an alternate explanation for dark matter.

The thing that led me to ask this question is reading about Jupiter's magnetic field interactions with the plasma emitted by IO.  Jupiter's magnetic field forces the plasma to orbit Jupiter about as fast as Jupiter spins, and I am wondering if likewise, the supermassive black hole at the center of our galaxy "herds" the rest of the galaxy in a similar manner as per the article and image below.

http://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter#Role_of_Io


texts: Have we observed any rogue/wandering stars?

texts:
Do we know (have we observed and cataloged) any rogue star, being not part of galaxy, but drifting somewhere in inter-galactic space?

I know that determining if a star is a part of galaxy or not is a matter of definition, but let's take those, that inter-galactic space is where the gravitational influence of any galaxy isn't dominating.


texts:
The Moon orbits the Earth.
The Earth orbits the Sun.

Does the Sun orbit another bigger star?
If so, does this star orbit, in turn, a very big star?
... etc ...

What are all the intermediate subsystems up to motion around the center of the Milky Way?


texts:
I think that the answer to this question will be useful as a scale to appreciate the size of the universe.

So, If we count Avogadro's Number of stars that are closest to Earth, how big that space would be? And how many galaxies/clusters will be engulfed in that space? And for a start, let's just consider only three dimensions.


texts:
In science fiction, when a starship captain commands "all stop," the audience simply assumes that, like a terrestrial naval ship anchored to the ocean floor, the ship literally comes to a complete stop.

Does our current understanding of celestial mechanics provide a meaningful definition of a complete stop in space — for any object?


texts: Which star has the highest mass loss rate?

texts: We have known lists of stars with high luminosity, temperature, mass, size, and much more. However, I have not seen a list that compares each star's mass loss rates. So, which star (or stars) have the highest mass loss rates, and is it going to be more likely to be an LBV, RSG, or a WR star?


texts: If the hammer and feather move at the same speed why do comet and the tail particles move at different speeds?

texts:
From this exhibition by an astronaut on the moon dropping a hammer and a feather and showing they move at the same speed, why do the particles of the comets tail move away from the comet, slow down and fall away as it disintegrates? Shouldn't they move at the same speed of the comet?
For example if the feather was attached to the hammer and it was falling and broke off wouldn't it fall at the same speed, not start to trail behind and eventually fall away?


texts: Where did the hydrogen come from in a type II supernova?

texts: Type II supernovae have hydrogen. Where did the hydrogen come from if that is the first element used up in the star's life cycle? Also if our solar system was seeded from an ancient supernova where did the sun's hydrogen come from?


texts: Does conservation of energy make black holes impossible?

texts:
I was musing today about black holes, and came across what seems to be a contradictory fact about black holes. If any matter were to actually fall into a black hole, it seems like it would need to gain infinite energy.

This is seen by considering an object close to the event horizon of a black hole. If we consider escape velocity for a particle at the event horizon, it has an escape velocity of the speed of light. The energy required to achieve this escape velocity would therefore be infinite. As we approach the event horizon, it seems like we should be able, then, to define an arbitrarily small distance across which the particle gains an arbitrary amount of energy.

As the object approaches arbitrarily close to the event horizon, it seems like its relativistic mass would therefore grow arbitrarily high, resulting in a corresponding increase in the gravity of the black hole, which seems impossible.

If we extrapolate back to the formation of the black hole, it then seems like it should be impossible for any particles to fall into the black hole, even as it forms. Instead, mass should be distributed in a probability cloud whose density asymptotically approaches zero at the event horizon of the black hole. New particles could then enter this cloud of incredibly dense (but not black hole dense) matter, where they would be captured rather than falling past the event horizon, within which there would be a vacuum.

If this were true, there would be no black holes, only hollow spheres of radiation and matter which externally look like black holes, since their gravity would be great enough that particles entering the sphere would exit at an incredibly slow rate. Interestingly, such spheres wouldn't suffer from the apparent information paradox that black holes do.

That or I'm missing an obvious explanation for why that's all wrong, which seems more likely because I'm neither a physicist nor an astronomer. What's the critical flaw in this argument?


texts: when are you able to see Halley's comet, and for how long?

texts: I've been hearing about Halley's comet for a while now, and I'm just curious as to when and where to see it, and for how long. :)


texts: Why does Astronomy still use the Anno Domini system for Time Synchronization

texts:
Since Time is such a valuable variable in the calculation of Astronomic movement why does the Astronomy Community still rely on the the 6th Century Anno Domini system developed by Dionysius Exiguus to establish Time on Earth.
Obviously it is not 2013 on Earth, wouldn't it be more technically accurate to determine the actual time on Earth or get as close as possible to it than to use a time that is clearly wrong and developed by someone 15 centuries ago?


texts: Distance between Earth and where The Big Bang's happened?

texts:
The universe is around 13.8 billion years old.
We know how fast the light is.
We also know how fast our galaxy moving around inside observable universe.

Is it possible to calculating distance between Earth and the place where The Big Bang happened ? If so how far away our planet ? and is there a something special about this place ? (A black hole or something else really crazy?)


texts: Rendering stars in 3D space - AbsMag to OpenGL scale values

texts: I am using the AMNH's Digital Universe stars.speck file to render stars in 3D space. The speck contains parsec-scale coortinates, where our Sun is at 0,0,0. It also lists the AbsMag values - i.e the luminosity from 10 parsecs away. I want to now scale my star textures in openGL, so the resulting star sizes are accurate. How could I convert the AbsMag inverse logarithmic scale to a scale value of 0-infinity, where 0.0 means the star goes away, and 1.0 is no change? Obviously can't make the Sun stay at 1, as that would make it huge.


texts: Distinction between signal from terrestrial and signals from celestial sources

texts:
How is it confirmed that a radiation source (not in visible light) is either terrestrial or celestial?
If in-depth explanation and/or references are possible, I would be grateful.


texts: Where can I get J2000 epoch initial mean longitudes for all planets and celestial bodies?

texts: I'm following this paper to calculate geocentric coordinates. The authors have provided the initial mean longitude values for the planets (provided in attached table) for  01.01.2000 @ 00.00 UT but I'd like Uranus, Neptune, Pluto and some asteroids also. Is there anywhere this information is freely available or did they calculate it? Many thanks.


texts: How did matter first come into existence?

texts:
How did matter come to be?
Why is there matter if matter and antimatter cancel each other?


texts: Would bacteria on incoming meteors burn before impacting?

texts:
I was watching an old Ancient Aliens last night and there was a whole episode dedicated to how the "Gods" (aliens) were spreading disease/plagues.

A good portion of the show was dedicated to how aliens may be sending meteors to Earth with bacteria to spread disease.

Part of this was several scientists talking about an experiment with a rock set in orbit (forgive me that I don't have a link to this).  Said rock had many forms of bacteria on it and after a certain amount of time they grabbed the rock and confirmed that some bacteria can survive space conditions.

Then there were other scientists saying that some forms of bacteria/virus may even get worse in zero gravity or space conditions - used salmonella as an example.

But my question is does this even matter?  Wouldn't any bacteria or virus colonies on a meteorite just burn up in our atmosphere?


texts:
I have a doubt in Earth's curvature. This question may be dumb.

How much we need to travel on the Earth to feel the curvature of Earth? Hypothetical case: for example, we are building a vehicle. How much is the length of that vehicle to feel the curvature of the Earth?

A more question, what is about the speed of the Earth. Again a hypothetical case. If we are travelling in aeroplane from one country to other, whether we can take Earth's rotational speed (rotating around it's own axis), and aeroplane speeds as comparables and find the resultant speed?


texts: Is dust extinction/reddening caused by Rayleigh scattering or some other physics phenomenon (or both)?

texts: I know from Rayleigh scattering that in the atmosphere redder photons are scattered less than bluer photons by the particles that both make up and are floating around in the atmosphere.  This is why the sky is blue and sunsets are red.  My question is, is the physics behind Rayleigh scattering (which, according to wikipedia, is due to the electric polarizability of the molecules) the same physics behind dust extinction? Additionally, is it the only physics in play or is there additional physics that causes the phenomenon?


texts: How does 2015 RR245 compare to the other KBOs predicting "Planet 9"?

texts: 2015 RR245 was recently announced, in a highly eccentric orbit in the KBO. From what I understand, the so called "Planet 9" was predicted because these objects tended to be pointed in a particular direction. How is this object pointed relative to the other KBOs?


texts: How could we search for old cold neutron stars?

texts:
This is the inverse of Will Gaia detect inactive neutron stars? (to which the answer was "probably not").
So if Gaia can't do it, what would it take? How could such a search work, and what technological advances would be needed to have a reasonable chance of it working?


texts: So, you can determine the location of an object in an orbit at any given time via Kepler's laws, but how do you determine the orbital inclination at any given point in time?


texts: How do you figure out the magnitude of stars?

texts:
How do you figure out the star magnitude of a star? Do they just give it a guess or what?
If not, how do they figure it out accurately?


texts: How close would an LED have to be to your eyes to be as bright as the looking at the sun during midday?

texts: I wonder if it is possible to have an LED screen bright enough to simulate the brightness of our sun from Earth, and if so if it could be made small enough to fit on a laptop screen.


texts: How did early estimates of a "potato radius" set 1 eV ~ GMμ/R and get 200 to 300 km?

texts:
@DavidHammen's answer explains that Goblin has roughly a potato radius and I can't believe I just wrote Goblin and potato in a serious question. That happened.
The linked paper; The Potato Radius: a Lower Minimum Size for Dwarf Planets (Lineweaver & Norman 2010) says:

Derivation of the Potato Radius
One could begin, as the author did initially, by setting the gravitational acceleration at the surface of a body equal to the acceleration needed to undo the electronic bonds. But this is not what we want since it is equating surface gravity with the force needed to crush rocks. The Earth is spherical but its gravitational pull is not so strong that it crushes rock at the surface. One has to go down ~10 or ~20 kilometers before the overburden pressure can do that.
To our knowledge, the published derivation of something most analogous to the potato radius is in Chap. 1.2.1 of Stevenson (2009). Focusing on energy (not force or pressure) the electronic bond energy of ~ 1 eV is set equal to the gravitational energy ~ GMµ/R where µ is the mass of the particle in question and R is the radius of the object. The result is a radius of a few thousand kilometers for a rocky body and ~ 1000 km for an icy body. This was meant to be only an order of magnitude calculation. We try to improve on it below.

We have there
$$1 \text{eV} \approx \frac{GM\mu}{R} $$
1 eV is about 1.6E-19 Joules and $G$ is the gravitational constant, but how to I use this to arrive at a potato radius $R$ of about 200 to 300 km? I can read the words in the quote, but I don't understand how to proceed.


texts: The role of 'the interference of waves' in VLBIs'

texts:
I've been trying to find out how VLBIs work. It says in this book I've read that many radio telescopes around the world work in tandem to use the interference of waves to get a clearer picture of celestial bodies, which is how they got the black hole picture.
But what I've found only talks about the difference in the time that radio waves takes to arrive to different points on the globe and how that is calibrated. Not a single word about the interference of waves.
Wikipedia's Very Long Baseline Interferometry; Method has a short discussion, but does not really explain the basic principle of how delays and interference are used to build up an image.


texts:
Which rotational frame does the universe prefer?
We say that the earth rotates once per 24 hours.
But is this the master frame?
What is the solution to Newtons Bucket Problem?

I would like if the answer is a detailed explanation.
I know about high school math and science and participated in a astronomy olympiad.


texts:
When was it confirmed that Mercury has a 3:2 spin-orbit resonance and by whom (research group/radio observations...)? The first suggestion was made by Giuseppe Colombo in 1965.
Its proximity to the sun posed some problems in nailing the orbit and rotation of Mercury, but what made it eventually possible to measure it? Was there a breakthrough or development of a technology or was it just "steady scientific progress"?


texts: What Are Some Good Southern Hemisphere 7x50 Binocular Targets?

texts:
I've been interested in astronomy for a while but only recently purchased my first set of binoculars (7x50). I've been learning various constellations as well as spotting some other objects with the help of my planisphere. I live in the suburbs of a big Australian city, but far enough out that the light pollution isn't too bad.

I've already been looking at objects like the Orion Nebula and Pleiades, but hoped some experts might be able to give me a list of objects that would be worth hunting down with my binoculars. Ideally some that would be visible now (it's Summer here), but some for other times of the year would also be fine.


texts: Can a magnetic field of an object be stronger than its gravity?

texts: Can a planet, star or otherwise have a magnetic field that is stronger or have more range than its gravity?


texts: Tidally locked Venus, is it possible and consequences?

texts:
Suppose, Venus becomes tidally locked. Will its dark side cool enough so that the CO2 from the atmosphere to precipitate in liquid form to make an ocean?
Also, I wonder, how close Venus is to becoming tidally locked? Does it accelerate or slow down its rotation?


texts:
From a distance of 160,000 light years, the Large Magellanic Cloud looks at first glance like an isolated section of the Milky Way, running across southern Dorado into neighbouring Mensa.

Is the Large Magellanic Cloud contained in the Milky Way? I don't think it is, but I'm not entirely sure.


texts: Infrared Astronomy for non transiting objects?

texts: I understand how infrared spectroscopy works for determining molecules in the atmosphere of a transiting object (like Venus from Earth). But how does this work for objects that don't pass between us and the sun? If the IR spectra is an absorption spectra, is the light emitted by a planet, like Uranus, enough for us to determine its atmoshpere's composition via IR spectroscopy?


texts:
I recently heard in a discussion that the sun is not massive enough to self-ignite via core-pressure. It simply has not enough mass to "generate" the gravitational force needed for that.

The reason why the sun is still burning is that there is a really small chance that two Hydrogen-Atoms fuse without needing the activation-energy/the activation-energy being provided by a random effect like High-Energy-Particles. Even though the chance is really low, there are so many Hydrogen-Atoms that overall the chance is actually high (law of big numbers). Once some fusion started it provides enough energy to fusion more Hydrogen, leading to our sun shining.

The person who presented it only stated that there are sources, but sadly the ones I found from his material didn't support this (nor did they deny it). The sources where about other topic related to stars. I can't ask the person himself as he already changed the university.


texts: How to measure mass of planets' core from orbit

texts:
I am told in an astrophysics lecture the following.
The mass of Saturn's core was measured by Cassini when it completed its final flyby between the rings and the planet itself. It was also found that its core has a sharp boundary.
Juno measured the mass of Jupiter's core & found that its boundary is a bit fuzzy.
How were these measurements done in principle?
(Ie what was measured from which we can deduce core mass?)


texts:
I found this Wikipedia page on this property called color index. Unfortunately, I could not find a source that explains this property well.

My interpretation is that color index is the brightness difference of an object through some 2 different wavelengths. Is my understanding correct? If not, could someone explain it to me?

Also, if my understanding is correct, why would there need to be a calibration table, as shown in the above link?


texts: Are the Trappist-1 planets in stable orbits?

texts:
The Trappist-1 planets all orbit very close to each other.  During NASA's press release, they mentioned that these planets are close enough to disturb each others orbits.  Is this system stable over a long time scale?  Or could we perhaps just have imaged this system before one or more planets is ejected or destroyed?

Later into the press conference, one of the scientists says "these planets should have formed further out and migrated inwards".  Could they possibly still be moving inwards, or are their orbits now stable?


texts: Would Pluto keep an orbit without its moon?

texts: I haven't studied space that much, but I know that Pluto has very little gravity, but just enough to keep a human on its surface. Would Pluto keep a regular orbit around the sun without some extra support gravity, such as its moon?


texts: Surface height distribution for Jupiter's moon Io?

texts:
The moon Io has around 100 mountains:

These structures average $6 {\rm\, km}$ in height and reach a maximum of ${\rm 17.5 \pm 1.5\, km}$

I am now looking for the overall histogram, i.e. elevation distribution of Io's surface and/or a hypsometric curve for Io.
References

JPL: Topography of Io


texts: How are sulfuric acid clouds able to completely cover Venus?

texts:
It is a well-known fact that an opaque layer of highly reflective clouds of sulfuric acid completely shrouds Venus preventing its surface from being seen from space in visible light. This is mentioned in the official NASA site and also in some papers:

The atmosphere of Venus is made up mainly of carbon dioxide, and
thick clouds of sulfuric acid completely cover the planet. (nasa.gov)

The clouds that completely shroud the planet are located in the altitude range of approximately 50–70 km with upper and lower haze extending up to about 100 km and 30 km correspondingly, thus occupying the upper troposphere and the entire mesosphere.
Titov, D.V., Ignatiev, N.I., McGouldrick, K. et al. Clouds and Hazes of Venus. Space Sci Rev 214, 126 (2018). https://doi.org/10.1007/s11214-018-0552-z

The sky of Venus is fully covered by thick clouds of sulfuric acid that are located at a height of 45-70 km, making it hard to observe the planet's surface from Earth-based telescopes and orbiters circling Venus. (eurekalert.org)

But in this question, user @Cornelisinspace came to the conclusion that "there is much less sulfuric acid than water !".  So, if there is less sulfuric acid than water (which is already present in trace amount):

how is it able to "completely spread over" and "fully cover the planet"?

why is the cloud cover so thick (so thick that it reflects most of the solar radiation (except in some dark patches where it gets absorbed) making the planet opaque)?


texts: How do we know the current state of the universe by observing it from Earth?

texts: Since light travels at the constant speed and takes more than a billion years to travel from the distant parts of the Universe, how do we know the state of the distant regions? Do those still exist or not? Is our picture of the Universe outdated?


texts: Practicing image processing and analysis

texts: I'm just getting into image processing and analysis using Python, and I'm looking for astronomical images (preferably raw, unprocessed images) to practice on. Can anyone suggest some places I can download public-access telescope images from?


texts: Can "rock stars" form from interstellar dust?

texts:
Imagine a large and massive dust cloud made of solid phase micrometeoroids, asteroids and larger planetoids, all material in it is in solid phase, and contains no hydrogen or other volatiles in gas form - think about the chemical composition of inner solar system asteroids.

Can this debris cloud collapse and form a star that fuses the lighter than iron elements? What properties such object would have? Would it be very hot? Very cold?

Nowadays it's unlikely, but I can imagine situations like this near the end of the stelliferous era when most of the hydrogen is used up and the universe is filled with white dwarves, neutron stars, and black holes, and lots of dust from all the previous explosions of the supernovas and planetary nebulas of the last hydrogen stars.


texts: How to derive uncertainty on photometric magnitudes of extended objects?

texts: Suppose I have an astronomical image in some bandpass and I do photometry of an extended source (i.e., a galaxy). I measure an enclosed magnitude within my aperture or galaxy profile. What is a straightforward recipe for obtaining a realistic uncertainty on that magnitude? Can I just do something like 1/(S/N) or is that only for point source photometry?


texts: What can be learned from, or noted in this LIGO Orrery video?

texts:
The Ars Technica article Physicists detected gravitational waves from four new black-hole mergers notes that

  This brings the total number of events detected by LIGO and Virgo to 11.

It also has a link to the SXS Collaboration' YouTube video LIGO Orrery

SXS stands for Simulating eXtreme Spacetimes https://www.black-holes.org/

  The SXS project is a collaborative research effort involving multiple institutions. Our goal is the simulation of black holes and other extreme spacetimes to gain a better understanding of Relativity, and the physics of exotic objects in the distant cosmos.

so quite a substantial amount of math and physics went into this work.

But as an outsider I don't really understand what I'm looking at. The video's comments just say that the surface is the event horizon:

  Computer calculations modeling the gravitational waves LIGO has observed to date and the black holes that emitted the waves. The image shows the horizons of the black holes above the corresponding gravitational wave.

Question: What does the color represent, could it be some kind of "temperature" or energy density of distorted space on the event horizon surface? Why can I see some evidence of rotation in the resulting merged object for a few rotations, then it disappears? What else (if anything) can be learned from this video?

Screen shot from the YouTube video LIGO Orrery:


texts: How exactly does Hawking radiation decrease the mass of black holes?

texts:
From what I understand so far, when one of virtual particles crosses the event horizon and the other does not, they can not annihilate each other. The latter wanders off into the universe (btw, is it still virtual at this point, and what does 'virtual' mean at this point, if so?), while the other gets consumed by the black hole. I don't see how this event contributes to evaporation of the black hole (since the particles do not originate from the black hole). Shouldn't the consumed particle actually add-up to the black hole mass?

The closest question to mine is Does Hawking radiation in fact bring mass into the universe?, but I don't find the answers satisfactory.

I.e. "the escaped virtual particle is 'boosted' by black hole's gravitational field into becoming a real particle", rather adds to the question then answer it.

EDIT: I am humbled by the knowledge presented in the answers and feel incompetent to mark any as the most suitable. I hope this is okay.


texts: Why doesn't smashing larger and larger bodies incrementally into a rocky planet create a star?

texts: Let's start with a stationary Earth. If we smash a few Mercury-sized objects into the Earth, the Earth begins to accrete mass. Repeat this step with larger and larger objects, until we start impacting Jupiters into the growing Earth. If we cause an even larger planet to crash into the Earth (say, a 3-Jupiter mass object), we will eventually reach the boundary between planets and brown dwarfs. If we continue accreting more Jupiter-sized objects to the Earth, we will pass the brown-dwarf limit of 90 Jupiter masses, we will reach stellar masses. But instead of turning into a star, this object either stays molten as a "planet", blows itself apart, or even collapses into a black hole. I tested this in Universe Sandbox 2 about 20 times, and none of them coalesced into a star. Why does this happen, and would this be realistic?


texts: Jupiter appearing odd through a 6" reflector

texts: Recently I have been trying to get a good look at Jupiter through my 6" Orion reflector, but the image seems to always come out as an oblong white streak. I've tried changing the lenses I'm using, but that only changes the size of the oblong streak. I looked at the moon with the same lenses to see if it was a general problem with the reflector, but the image of the moon came out clear as usual. What sort of problems could be causing this?


texts: How to know the orbital and physical characteristics of the solar planets with only a non-professional telescope?

texts:
I have always been fascinated to see that with so little knowledge, scientists have succeeded in discovering the planets of the solar system, and have succeeded in defining them by their masses as well as by their orbits, temperatures, and so on.

I would like to try to trace this process myself but I do not know how to do.

My starting postulate is simple, I have at my disposal a simple telescope and DSLR camera.

My goal is to define the solar planets, ie to know their masses, their sizes, their orbits, their revolutions.

How to achieve it? Do you have any leads?

EDIT :
Thank you for your awesome answers and sorry for my late reply.

I think that I have to separate this problem into 3 different points :

earth's variable : I can find it by reproducing the Greek calculus
Sun's variable : with the two-body problem
planets' variable : with the Gauss' method & observation

I keep you update about my advancements ;)


texts: Extraction of error spectrum in Sloan Digital Sky Survey

texts:
I need to know does SDSS spectrum contains error spectrum as well?

http://www.sdss.org/dr12/spectro/spectro_basics/#AbouttheSpectra

The above link says, further HDU's contain error spectrum and than tells about the inverse variance. Does inverse variance contains error spectrum? If yes, how can I extract that, if not where can I find error spectrum?


texts: Looking for help in understanding how black holes can move

texts:
I'm looking for help in understanding how black holes can move through space. This is probably a trivial question for anyone who knows astronomy, but probably due to my lack of education on the topic, I can't come up with an answer or visualization on my own.

Specifically, my confusion is as follows:

I saw Dr. Kip Thorne in a documentary on black holes state that there is no longer any matter in a black hole once it forms. He said something like, the matter was there, but it has been crushed out of existence and basically has been transformed into the energy that produces such a great curvature of spacetime.
From that description, it seems to me that within the event horizon of a black hole it is simply this enormous "dent" in spacetime itself that is moving around.
I also saw Dr. Andrew Hamilton in another documentary state that within the event horizon of a black hole (again paraphrasing), space is falling toward the singularity so fast that it effectively drags everything with it (including light).
Based on 1 (no matter remains in the black hole) and 2 (space is being pulled towards the gravitational singularity from all directions), I try to picture the black hole moving, and it seems to me that if, say, the black hole moves in one direction, I can see how its event horizon is also moving, bringing that bit of space into it. But how can the other end of the black hole let space "escape" in order to complete the motion?

I am pretty sure I am just missing something here, because as I have learned, black holes have been found in galaxies and solar systems and many other places where they have to be moving. But I want to understand why it is that space itself can cross the event horizon when other things can't. And along those lines, it seems amazing to me that a black hole could be in essence a kind of scar in spacetime that can also move through it.

Thanks for any help and sorry for the wordy question.

UPDATE: Thanks for the answers so far. I hope someday I'll be able to comprehend them fully. :-)

I would like to add some further context that better illustrates my question.

If you go to the video here and watch the first minute or so (up to around 31:00)

http://youtu.be/Niurc_6xIYg?t=29m48s

you can see Andrew Hamilton explaining what I have a question about along with an animation to illustrate the description.

On the one hand, we have spacetime curved in such a way that the black hole structure remains intact - that is, there is a constant event horizon generated by that extreme curvature. But on the other hand, Dr. Hamilton describes a flow of spacetime between the event horizon and the inner horizon, and he says it is plausible that within that area, space itself (which has no substance, in his words) can be described as flowing in and being flung back out.

I don't understand how space can have these two opposing properties - (1) a curvature that is maintained in order for the black hole to exist, that is, a curvature generated because matter can interact with the space and (2) a flow of space that sounds like it is not subject to a restriction of crossing back out of the event horizon.

It's that apparent difference between (1) and (2) that I want to understand better.

Thanks!


texts:
By what process can a user running the latest version of OS X Yosemite execute MOOG with $MOOG in bash?

Due to the removal of 32bit libraries in OS X 10.7+, the Fortran77 code for the MOOG installer cannot be compiled on OS X 10.7+. As a result, the pip installer will not work.

I have followed the instructions in the above guide, except that:

I cannot install scisoft because the link in the page does not work
I have supermongo installed independently of MOOG, so $sm works for me
I have made my computer compile Fortran code with the true version of gcc, not Apple gcc.

Unfortunately, in addition to being unable to compile the code, the pre-compiled binaries aren't working for me right now on Yosemite 10.10.4. The error I get when I $MOOG is:

open: No such file or directory
apparent state: unit 35 named /.moog/Barklem.dat
lately writing sequential formatted external IO
Abort trap: 6

But I do have .moog in my home folder, and it has the Barklem files inside, and both the directory and files have been made readable/writable with:

$sudo chmod +rw .moog and $sudo chmod +rw Bark*

Motivation: Astronomers try to solve the problem independently
Justification: AstronomySE topics include: "Setting up, using and maintaining your astronomy related equipment"


texts: What does "negligible mass" mean in the formulation of geodesics equation?

texts:
Wikipedia says:

  According to Einstein's theory of general relativity, particles of
  negligible mass travel along geodesics in the space-time.

Does that "negligible mass" only refer to particles or objects like a spacecraft, whose mass are literally negligible to that of a black hole, are also taken into account?

If the mass of a spacecraft is not negligible, how has one to modify the geodesics equation corresponding to that mass?


texts: How frequently are other planets in the Solar system hit by meteorites? For example, is the rate per unit area about the same?


texts: What does an Einstein-De Sitter universe look like?

texts: I know an Einstein-De Sitter universe is a "flat" universe, i.e. with a K ("bendiness", sorry, don't really know what to call it in English) factor of 0, but does that entail anything as to its contents? The Wikipedia page is quite lackluster on this aspect, and my astronomy course textbook merely slightly less so (it says it's made up mainly of "non-relativistic, dust-like matter").


texts: Are other planets' gravity measurable on Earth?

texts: I understand that we can calculate the gravitational force  other planets exert on Earth, but can it be measured?


texts: What happened to NS spectral types after OBAFGKM?

texts: What happened to NS after OBAFGKM? The full mnemonic was "O Be a Fine Girl Kiss Me Now Sweetheart" a few decades ago as I remember. I think it was that way in "The Larousse Encyclopaedia of Astronomy" which I once had. The Cambridge Encyclopaedia of Astronomy (which I still have, first published in 1977, Reprinted 1979) has "OBAFGKMRNS", this seems to stem from work done by Annie J. Cannon resulting in the classification of over 225,000 stars in catalogues published between 1918 and 1924, suggesting that "OBAFGKMRNS" is the accepted list "now" (1977?). So what happened to the "R" also? Of course you can't quote these mnemonics now, because of "Political Correctness", though you can always explain that G could be either Guy or Girl!


texts: Are new stars less pure as generations go by?

texts:
If stars are primarily made of hydrogen, which is then burned to helium, and then on and on and on down the chain until you either reach iron, or in extreme cases much heavier metals, this then implies that as time goes on, more and more heavy elements will disperse into the universe.
Now, the universe is huge, and there's still a lot of hydrogen left to make stars out of.
That being said, in denser star-forming regions, much of this gas is available for recycling as the remnants of old stars get turned into new ones.
Doesn't this all this imply that as time goes on, more and more heavy metals will be included within young stars from the start?
It may be a very small percentile, sure, but it I assume it would increase slowly as the generations go by.
Furthermore, I assume many rocks get swallowed up by a star during the formation of the planetary disc, which likely adds to the impurity.
Does any of this have any adverse effect on the lifespan of the sun? Or are we talking quantities so small in objects so huge in time-spans so large, that even a dip of a hundred years is completely meaningless?


texts: Does lightning on Venus affect the chemical composition of the clouds?

texts:
It is established that lightning occurs on Venus due to the presence of sulfuric acid clouds. Does this constant electric discharges and possible electrolysis affects the composition of the clouds on Venus for instance, sulfuric acid dissociating into ions like hydrogen ions, sulfate/bisulfate ions or decomposing into sulfur oxides or changing to other sulfur oxoacids?
There was a question on space.SE that discussed if peroxydisulfuric acid (H2S2O8) created due to electrolysis of concentrated sulfuric acid (from the lightning) can be used as a propellant fuel. If the species is indeed formed in the atmosphere, how abundant is it? How much of this species is present in the atmosphere?
Does lightning on Venus change/affect the chemical composition of the clouds? Does it affect the sulfur cycle of Venus in any way (so that it could create more exotic sulfur species)?


texts: Any possibility for life on moons of planets in our Solar System

texts: We all focused on our moon, to find water or life, or any thing related to life. But my question is, is there any chance for life in moons of other planets in our solar system, others have too many moons....


texts:
Many craters on Moon have circular shape as a result of the explosion upon impact. I got interested in oblong craters created by impacts striking the surface at a very low angle. Is there any statistics on the number of elongated craters on the Moon available?
An article on an Oval Crater captured by the Lunar Reconnaissance Orbiter states:

Non-circular (oval or elliptical) impact craters can form when the impacting bolide trajectory to the surface is <15° from horizontal or when the bolide impacts a sloped region on the (or some combination of both factors).

How well-studied is that threshold impact angle? Is there a phase transition from no explosion if below 15°, to explosion if impact angle is steeper?
References

Are all impact craters circular?
Lunar and Planetary Institute: Shaping the Planets: Impact Cratering


texts: Why is JD 2451545.0 = January 1, 2000, Noon instead of JD 2451558.0?

texts:
I know that JD0.0 is Jan. 1st 4713 BC at Noon, a year in the Julian calendar is 365.25 days and that the number after 'JD' is the number of days so...

I want to calculate the number of days between Jan1.2000AD and Jan1.4713BC and I do

$$ \text{# of years} = 4712+2000=6712 \text{ years (skipping the year 0)} $$
$$ \text{# of days} = 6712 \cdot 365.25 =2451558 $$

So I think it should be JD2451558. An extra 13 days. What's going on here? I thought maybe the length of a year in the Julian calendar is $2451545/6712=365.2480632\text{days}$ but I can't find anything to support that.

I imagine that people would have questioned whoever proposed J2000 as JD2451545.0 and got a satisfying answer but I can't find one! Help


texts: Luminosity of the Milky Way compared to Seyfert galaxies

texts:
This page lists the total luminosity of the Milky Way (MW) Galaxy to be $4\times10^{10}$ $L_{\rm sol}$, and $L_{\rm sol}=3.86 \cdot 10^{33} {\rm ergs.sec}^{-1}$; this gives a total luminosity for the MW of $1.54\cdot 10^{44}{\rm ergs.sec}^{-1}$.
But Seyfert galaxies can have luminosities of $\sim 10^{40} - 10^{42}{\rm ergs.sec}^{-1}$, and this is due to the high luminosity of the galactic core of a Seyfert.
Since the MW doesn't have an active SMBH, does this mean that the MW is particularly bright due to a high luminosity stellar population? Or am I missing something?


texts:
I was wondering about what happens when a black hole forms.

I mean, a black hole forms when a star collapses, so when an enormous mass of protons, electrons, neutrons etc collapses together.
But when it does, all those particles should come "in contact", so what happens when electrons and protons come in contact?
Shouldn't they annihilate each other releasing energy?
In the end, shouldn't the singularity be composed only by the mass of particles without an electrical charge?

I'm not a professional, I just find astronomy fascinating and I like to read about it, so forgive me if I said something obviously wrong :)


texts: Orion Skyquest xt8i Intelliscope Dobsonian Telescope Lens

texts: I have the telescope in the title, which camps with 10mm and 25mm eyepieces. Both cannot really see anything in detail past the moon. All planets look like stars. What are good eyepieces for seeing all the other planets in detail, respectively?


texts: How far away is the nearest compact star remnant likely to be?

texts:
Neutron stars and black holes are hard to detect when they are solitary, and there seems to be big uncertainties about how common they are. White dwarfs are much easier to detect and the nearest one is Sirius B only 2.6 parsec from here. Should we expect to have exotic company even closer by? How likely is it that we have a yet undetected compact star remnant nearby, as in as close as our nearest active star? What are the odds of spotting one of them nearby?

How could a yet undetected one be discovered? Could one of the upcoming sky surveying telescopes catch it or would one have to rely on a rare microlensing event? How would it then be observed? Would such an exotic object, say, just one parsec away, give important insights in physics given its relativistic effects and strange composition?


texts:
So I am trying to write a research paper (about 3700 words) for high school about the topic "The dominance of Gravity vs Strength and its dependency on crater size" or something similar.

I have found the following information which I am struggling to understand and has been holding me back from continuing the research : (Can someone explain what dominance is and how can I approach reading more about this topic?)

Making a crater requires both material strength of the target and gravity to be overcome.
which of these dominates depends on the size of the crater

$$ \rho g d = \frac{1}{5} \rho g D = Y $$

Where Y is the yield strength, ρ is density of the target material, d is the crater's depth and D is its diameter.

A related graph to this topic is this (Holsapple 1993) :

So please, can someone explain what all this means in simple terms ?


texts: Why aren't globular clusters disk-shaped?

texts: I thought that solar systems and galaxies are disk-shaped because that is the most stable shape under gravitation. Globular clusters are very old, often times older than their host galaxies, so why haven't they flattened out?


texts: What EM spectrum can stars be most cheaply observed using Remote Sensing during the day?

texts:
I would like to augment my GPS navigation with remote sensing of 'the sky.'  I am looking for any astronomical body that can be perceived during the day.

Maybe Pulsars emit something easily detected, seems IR is obvious..

Any input is appreciated.


texts: When does a solar eclipse become noticeable?

texts:
For a human viewer located on Earth, is there an approximate apparent magnitude at which a solar eclipse becomes apparent in the form of a noticeable change in ambient brightness?
Assume normal atmospheric conditions, etc.
Note, I am not interested in the noticeability of the Moon covering the Sun, but whether someone outside would think, "Hey, it looks darker than usual today!"


texts: Telescope size to detect Ceres (Newtonian)

texts: What is the minimum size telescope required to enable clear visual detection of Ceres?


texts:
This comment to Did nobody in the Astronomy community think 12,000 new satellites in LEO might be a problem? links to Phys.org's New ESO study evaluates impact of satellite constellations on astronomical observations which includes ESO's Areas of the sky most affected by satellite constellations shown below.
It's a fisheye lens view (FOV wider than 180 degrees)looking straight up.
In Earth Science SE I've asked What causes this arc in the night sky where the background is brighter on one side than the other? but here I'd just like to ask about the effect of sky brightness associated with the Moon on observational astronomy.
Question: Which kinds of astronomical observations most need to avoid the Moon being up? Are some observations relatively insensitive to the Moon being in the sky but not necessarily nearby and others negatively impacted or impossible because of it?

This annotated image shows the night sky at ESO's Paranal Observatory around twilight, about 90 minutes before sunrise. The blue lines mark degrees of elevation above the horizon.
A new ESO study looking into the impact of satellite constellations on astronomical observations shows that up to about 100 satellites could be bright enough to be visible with the naked eye during twilight hours (magnitude 5–6 or brighter). The vast majority of these, their locations marked with small green circles in the image, would be low in the sky, below about 30 degrees elevation, and/or would be rather faint. Only a few satellites, their locations marked in red, would be above 30 degrees of the horizon — the part of the sky where most astronomical observations take place — and be relatively bright (magnitude of about 3–4). For comparison, Polaris, the North Star, has a magnitude of 2, which is 2.5 times brighter than an object of magnitude 3.
The number of visible satellites plummets towards the middle of the night when more satellites fall into the shadow of the Earth, represented by the dark area on the left of the image. Satellites within the Earth's shadow are invisible.
Crediti: ESO/Y. Beletsky/L. Calçada


texts: What allows WFIRST to have similar resolution to Hubble over a 100x larger solid angle?

texts:
In the video linked in the Space.com article What Would It Mean for Astronomers If the WFIRST Space Telescope Is Killed?, (available in YouTube as WFIRST: The Best of Both Worlds) after 02:00 NASA astrophysicist Neil Gehrels says of WFIRST:

  It has the same image precision and power as the Hubble Space Telescope, but with one-hundred times the area of sky that is used.

Looking at the crude drawing on this NASA page shows only a Cassegrain-like geometry, but there must be additional goodies to obtain a similar resolution with a 10x wider field. Does it have three, four, or five mirrors like the E-ELT or the LSST (cf. What is a quaternary mirror and why does the E-ELT need one? and answers therein) or corrector lenses? What is the magic sauce that makes this possible, and is there at least a schematic diagram of the optics somewhere?

above: From NASA.


texts: Why is the Oort cloud presumed to be spherical?

texts:
Most descriptions of the Oort cloud depict it as a mostly spherical distribution of planetesimals, with occasional allowance for an inner component that is more donut-shaped. This is slightly at odds with the fact that most protoplanetary clouds and their derivative objects - planets, asteroids, comets and dust - will collapse to a fairly well-defined plane relatively early in a stellar system's evolution.

What evidence is used to postulate this? Does it come from numerical simulations of the solar system? Or does it help account for observed orbital inclinations of real comets?


texts: What is the current accepted theory as to why Titan has retained its atmosphere?

texts:
Titan (moon of Saturn) is unique in that it possesses a very thick atmosphere.  However, Titan is certainly is not the largest of the moons - Ganymede being larger.

What is the current accepted theory as to why Titan has retained its thick atmosphere?


texts: How can neutron stars have gaseous atmospheres?

texts:
Neutron stars can have small atmospheres. However, they also have extremely strong gravitationally pulls. Shouldn't the all the gas molecules be drawn to the star's surface, and become solids under the immense pressure?

Maybe I'm thinking about this the wrong way, but I don't see how it could be possible.


texts: Is extra-planetary phosphine actually a new discovery?

texts:
For all the buzz on Venus’s phosphine, I’m curious as to what sets it apart from the findings of phosphine on Jupiter,  Titan, etc.  There are several papers on NASA.gov dating to 1979 and more recent on the subject.   Per request in comments:  https://astrobiology.nasa.gov/nai/annual-reports/2010/jpl-titan/the-commonality-of-life-in-the-universe/index.html
Why is this latest finding of Venusian phosphine a special case?


texts: Intuitive explanation for why the universe is flat

texts: I went through Prof. Hitoshi Murayama's online lecture where he describes why the universe is flat. One of the evidences for a flat universe is the evenness of CMB radiation on all directions. My understanding is, that a uniform CMB pattern only indicates that inflation or early expansion of the universe was more or less equal on all directions. Why is this cited for the claim that the universe is flat, or in what sense is the universe 'flat'?


texts: What are future steps that can be taken in order to learn more about the inner workings of a Neutron Star such as "nuclear pasta" and the materials in the inner core?


texts: Is the radius used in the formula for the escape velocity the average radius of the celestial object or the radius at the starting location?

texts:
I learnt that the escape velocity is given by $$v_e = \sqrt{\frac{2GM}{r}}$$

Say I want to launch a rocket from the earth into space and want to calculate the escape velocity $v_e$ (I guess without air resistance). Which value of $r$ do I have to use:

The mean radius of the earth...
... or the distance between the center of mass of the earth and the launch location?

Thanks in advance.

(I have basic classical (and quantum) mechanics knowledge, but I am very new to orbital mechanics.)


texts: Is the equinox Sun always at the same zodiac sign in the ecliptic belt?

texts:
During the year the Sun moves along the ecliptic passing all the signs of the zodiac belt, in my understanding. This means that it takes ~1 month to go from one zodiac sign to the next.

The equinoxes are approximately on 20th of March and 23rd of September. This means the vernal equinox Sun is in Pisces (or Aries) and the autumnal equinox Sun is in Libra (or Virgo).

But also I found that there is the observation of precession of the equinoxes, where the equinox Sun passes through all zodiac signs in a calendar round that lasts ~25,000 years.

How is this possible?

If my understanding about the Sun's yearly round across the zodiac is correct, the equinox Sun should always be at the same zodiac signs. How can this precession happen?


texts:
My name is Chris, I am a junior and I'm taking an Astronomy class in my high school.
For a homework assignment, I have to explain how the Sun would move across the sky from September 1st through September 30th (assuming that the weather allows viewing)

For example:
In this picture the

sun is facing west.
If I were to take a picture of this same area at the same time every day for the whole month,
How would the sun change? Would it go east?
Would it go lower in the sky?

I, being a new student to Astronomy, honestly have no clue.

Thank you and if you need any more info please leave a comment.


texts: What's the smallest scale at which dark matter has been measured?

texts:
What is the smallest scale at which we have detected "dark matter" (more kinetic energy than would be predicted)?

If you don't mind indulging a follow-up: Would its detection on smaller scales be beneficial at all to advancing our understanding?

(aside: I base this question purely on the notion that our instruments have surely improved a bit since the 1930s.)


texts: What Types of Radiation Emanate in the Future and are Perceived in the Present?

texts:
Years ago I came upon a scientific text which mentioned different types of radiation and how they are perceived. One such form of radiation was described (from our perspective and understanding of space-time) to originate from the future and travel backwards through time.

I cannot for the life of me remember what this type of radiation was called and I would love to know it.


texts: What makes some meteor showers continue for days, while the "Unicorn shower" can be shorter than one hour?

texts:
The Washington Post's One hundred meteors in 15 minutes? ‘Short-lived outburst’ of shooting stars forecast for Thursday night. says:

A “short-lived outburst” is forecast for Thursday evening around 11:50 p.m. Eastern.

and later:

The prediction of an outburst comes from NASA research scientist Peter Jenniskens and Finnish Fireball Network’s Esko Lyytinen and was published in MeteorNews. The pair specialize in sniffing out meteor outbursts and storms, expertly calculating the orbits of various celestial bodies that give rise to meteor showers. Scientists still haven’t pinned down the object depositing the debris anticipated to trigger this display.
However, outbursts in 1925, 1935, 1985 and 1995 offered enough information for the team to produce a model. It indicates the close passage of a dense pocket of spaceborne debris. That narrow but potent debris stream is likely to manifest in a 15-to-40-minute-long barrage of meteors around 11:50 p.m. Eastern on Thursday, according to the researchers.
What will it look like? Jenniskens and Lyytinen indicate events in 1985 and 1995 produced 700 and 400 meteors per hour and wrote that this event could emit anywhere from around 100 per hour to even more than 1,000 per hour, the latter considered a meteor “storm.”
The outburst is calculated to last a fraction of an hour, so those numbers can be sliced in half. And because the radiant point of the meteors will be comparatively low in the sky, we can trim off a bit more.

The linked Meteor News article LIKELY ALPHA MONOCEROTIDS (AMO#246) OUTBURST ON THE MORNING OF NOVEMBER 22, 2019 gives orbital elements derived by different groups of astronomers that can be used to predict the time of maximum.
But what is it that makes this debris trail so narrow that the shower is predicted to last less than one hour while other showers can sometimes extend for several days?


texts: Is there an objective difference between space expansion and reduction in speed of light

texts:
I just read an article about the cosmological model of an inflationary universe. As you probably know it basically states that space itself is subject to an expansion over time.

Since there is no moving matter or energy involved, this might even happen at a "speed" faster than light. So far, so good and obscure. What strikes me is the principle that the speed of light as a fundamental constant can only be expressed as a function of space-time. Where do we know that the one is constant but the other can suddenly be variable?

Is there any reason why the point of view of an expanding space is preferred over, say, a reduction in the speed of light or an increase in the "speed" of time? Is there any objective difference or is it just the good old rubber metaphor being stretched (pun intended) too far?


texts: Max. speed that a space probe can communite with the Earth?

texts:
Question: Which is the average or maximum speed of a telematically sent message that a space probe can send? Is as simple as radio waves travel at the speed of light in a vacuum?

Context: I have been asked to make a first estimation about how much time is needed to send a space probe and then receive the signal.

I am supposing that the space probe is sent at $15 \ km/s$ but I have not idea of the telematic signal speed.

Thank you in advance!


texts: Is the moon "perfectly" tidally locked and, if not, how long would it take us to observe it's rotation?

texts:
I have a general understanding of how and why a body in space can be tidally locked to it's planet or sun and I'm aware that our moon is in such a state.

My question is, if our moon once had a rotation, has it slowed down to a point where across the course of our normal lifetimes we can't observe it's spin but across the span of hundreds, if not thousands, of years we could observe its current rotation?  In other words, if it has a rotation and if I looked at the moon today and jumped into the future, how far would I have to go to see a noticeable difference?

If the question is to relative then I'll ask it like this-- at the moons current rate of rotational speed and deceleration, how long would it take for the moon to rotate, say, 15 degrees on it's current axis?  I assume this would be enough to make the moon look "different" to the naked eye so I'll go with that concrete figure.

...or is the moon in either a state of near-equilibrium or is it "wobbling" due to other forces outside the gravitational pull of the earth (e.g. pull of the sun, asteroid bombardment, comets passing by, etc.) having minor effects on its rotational state and thus making it's rotation inconsistent and unobservable?


texts: If an asteroid twice the size of Earth passed super close would half of the Earth be pulled towards it?

texts:
So let's say that an asteroid the twice the size of Earth passed by going relatively slow say in between the Moon and Earth.

Among so many other things (and perhaps you can touch on some of the biggest 'other' things), would anything on the asteroid's side of Earth be essentially 'sucked' towards the asteroid. That is if I am standing outside and it passes would I lift off the ground and float towards it?

Conversely, would things on the opposite side experience two times the standard weight? That is if I am standing on the other side, would I feel heavier as the asteroid passes?

I realise there would be many other devastating effects but I'm most interested in this one...

The possible answer I suspect is that objects on Earth will not 'feel' anything as the Earth as a whole will move towards the asteroid and not every little thing inside Earth...


texts: False-color wavelength assignments in this "drop-dead gorgeous" image of NGC 2903?

texts:
C|Net's Hubble spots drop-dead gorgeous spiral galaxy tucked into Leo links to NASA's Hubble Spots Stunning Spiral Galaxy which shows the image below.

The caption on the NASA page doesn't mention the color coding, nor does it mention a reference to a technical page for the image's construction or history.

Question: Is this close to a "straight RGB" image of NGC 2903, or were filters used at certain wavelengths, possibly including IR or UV, in order to highlight certain regions and emissions, then assigned false colors. If the latter, is it a standard color coding?

The NASA page says:

  NGC 2903 is located about 30 million light-years away in the constellation of Leo (the Lion), and was studied as part of a Hubble survey of the central regions of roughly 145 nearby disk galaxies. This study aimed to help astronomers better understand the relationship between the black holes that lurk at the cores of galaxies like these, and the rugby-ball-shaped bulge of stars, gas and dust at the galaxy’s center — such as that seen in this image. Text credit: ESA (European Space Agency)

Click to view full size!


texts: Is having a surname which begins with the letter “A” a big advantage for a researcher in astronomy/astrophysics? Is changing our surname a good idea?

texts:
Is having a surname which begins with the letter “A” a big advantage for a researcher in astronomy/astrophysics?

If I have a surname which begins with the letter “M”, and I haven’t published any paper yet, would changing my surname to a name which begins with the letter “A” be a good idea? Or would the advantage gained be too small to make it worth the effort?

Approximately what percentages of research papers in astronomy/astrophysics order their authors …

purely alphabetically?
half by contribution and half alphabetically?
purely by contribution?


texts: Does the spin rate of a black hole correspond to a physical feature?

texts:
If the mass and orbital period for two neutron stars is known before collapse to a black hole and the rate of spin is known after collapse, 100 revolutions per second for instance, that will correlate mathmatically to a specific radius.

Does this radius correlate to a physical feature of the black hole such as a surface? Does this radius fall between the radius of the physical substance and the Schwarzschild radius? Is it the largest, intermediate, or the smallest of these?


texts: How to model and subtract nebular emission lines from stellar spectrum

texts:
I have optical stellar spectra from a star forming region with spatially varying nebular background.

I want to subtract the nebular lines from the stellar spectra (which are varying in intensity from spectrum to spectrum because of the varying background).  How can I model and subtract individual nebular lines from a stellar spectrum?  Are there any known tools for this?  Any help or suggestions would be highly appreciated.


texts: how to define the universe itself is expanding greater than than the speed of light?

texts:
since we observe galaxies and other steller objects in very tiny tiny scale, and how we can understand that the speed of expansion on universe is higher than the speed of light ?
even using the powerful telescope we just see the lights from a galaxy not the object itself in real time, also the lights are red shifted as well. So what's the basis of proving univers's expansion rate is higher than of light speed?


texts: In Orion, how close in local distance are the closest of the member stars?

texts:
All the members of Orion are within the Milky way, and some of them look pretty close to a neighbor. And could the bow* be an effect similar to the Radcliffe Wave?

*shield


texts: How did VLT's adaptive optics obtain this resolution for Neptune? Is it really working in visible wavelengths?

texts:
This image of Neptune taken with the VLT is really impressive. The resolution is achieved by recent improvements in the adaptive optics.

Gizmodo: New Super-Crisp Images of Neptune Show How Far Our Telescopes Have Come
Space.com Telescope Upgrade Produces Stunningly Clear Views of Space

But I don't really understand, is this coming from just one 8 meter mirror, or is this using more than one of the four that make up the VLT? Also, is this really using adaptive optics that work in the visible, including blue wavelengths? Or is this a false-color image? See the excellent answer(s) to Why (actually) aren't ground-based observatories using adaptive optics for visible wavelengths?


texts: How far does the moon move relative to the sun during a solar eclipse

texts:
Let's assume 'solar eclipse' occurred

Now, at what degree 'moon' will be in 'with reference to' earth (and)   at what degree 'moon' will be in 'with reference to' sun

In other words, by 'how many degrees' (out of 360 deg) the moon would have orbited 'earth' and 'sun' in the event of 'solar eclipse'?


texts: Venus transiting behind the Earth’s moon December 7, 2015

texts: Observing from Earth what are the odds that Venus does not line up in conjunction with Earth’s moon and does not transit behind the moon but slightly below or above with a complete visual of Venus during the entire conjunction of the two? What are the chances of a miscalculation in this event?


texts: Approximate formula to find velocity from cosmological redshift

texts:
From IOAA 2013 (Greece) Theory question no. 15, they stated that the approximate formula to find velocity from cosmological redshift is $$v = c*log_e (1+z)$$ and that it is often used by cosmologists. I did a quick google search but found nothing similar to this formula.
So, where did this equation come from and is it really often used by cosmologists?


texts: Is communication with proxima b possible with todays technology?

texts:
Let's say we have already a base on an exoplanet (like Proxima B). Would it be possible to send information from that base to the earth with todays technology? Not to talk about communication with such a distance ...

And how would it be done? Light or Radiowaves? Would it be possible to direct it so well, that some very small fraction of that wave could reach the earth? Or is it only a question of how much energy is needed?


texts:
I'm interested, is there any database of stars which I could browse on the internet?
It would be good, because I could read their parameters, features and maybe find a picture about it.
Thanks!


texts:
I'm making a very basic orbit simulator in C# using the Helix Toolkit.

I want to position the Helix lighting object at the sun, but also be able to calculate solar eclipse by the moon and earth for a satellite (position defined in ECEF).

Upon some brief research, I'm reading that getting an accurate position calculation for the sun in ECEF is a lot of work.

Is there an algorithm that can perform this calculation for me that I can include into an UpdateSunPosition() method so that I can have a reasonably accurate position calculation of the sun in ECEF?

Perhaps I have to scratch the eclipse calculation ambitions for now, instead calculate Geographical Position of the sun to get the Declination and Greenwich Hour Angle, Call those latitude and longitude, and then just give it a satisfactory altitude.

I've noticed that people are referring to SOFA a lot when talking about this, but I'm not sure what SOFA is, or whether I can use it in C#. I'm pretty sure it doesn't operate in a .NET environment like visual studio, but please correct me if I'm wrong.

I appreciate any and all suggestions or insights anyone may be able to offer here.


texts: Is there likely other significant objects we haven’t discovered in the solar system?

texts:
In the last few weeks, I read a few news that presented new advances in our understanding of the solar system, which implied we don’t really know our neighboorhood that well. I was wondering, how likely is it that we make a significant discovery around us in the next decade? Like discovering a new massive object (moon or even planet etc.), or that a planet area is habitable

I was assuming it’s very unlikely because we have good measures and models (gravity based or other means I guess) but those few papers (sorry I didn’t bookmark them) have me wonder. How much do we really know of our nearest environment, before looking at interstellar stuff?


texts: How long New Horizons will remain operational?

texts: As of now the New Horizons probe is well beyond the orbit of Pluto, moving towards the Kuiper belt. In an article I read that New Horizons will eventually follow the path of Voyager probes to the interstellar space. But how long it will remain functional and as further it will travel away from us it will also required very long time to receive data from the probe. Is there any pre-planned fate of the probe ?


texts: Why is time considered a (fourth) dimension?

texts:
Since dimensions usually refer to space and we naturally acknowledge three of them, we also perceive time and we separate space from time ("space AND time"), why is it, that time has ended up in the same shelf as spatial dimensions. Is time really a dimension "by nature"? Or is it a concesus, shifting our natural understanding what dimensions are, for practical reasons, similarly to defining particles?

EDIT: by Is time really a dimension "by nature"? I asked if there are good intrinsic reasons, given by to label time as a dimension, as opposed to us labeling it as such, to make it more tangible, explainable.


texts: How to compute satellite coordinates (lat,long) given antenna's coordinates, angles and satellite height

texts:
Given an Earth station (antenna) coordinates (in lat,long) ant its Elevation (El) and Azimuth (Az), how to compute the satellite coordinates (lat,long), known its height?

For simplification purposes, the Earth can be considered as an sphere.

So its given:

Earth Radius = 6371000 meters
Satellite Height, in meters (perpendicular to the Earth's)

Station lat, long (as parameters)
Station height (0 = Zero, for simplification purposes)
Station Elevation (angle, radius or degrees)
Station Azimuth (angle, radius or degrees)

In summary: I'm trying to calculate satellite's coordinates projected on celestial sphere. I.e. it's current ground track position from the horizontal coordinate system values and the position of the observation.

The problem can be shown in this two figures:


texts: Why is it that night has a almost a bluish tint to it instead of red?

texts: From my understanding/been told, the color red has the ability to travel the farthest in our atmosphere, being the reason why we have red sunsets and such. But with this in mind, why is it that at night the moonlight we receive seems to have a slight blue tint to it, and since red can travel the best out of our visible spectrum, how come our moonlit nights don't have a red tint instead of a blue one?


texts: Mapping selenographic coordinates onto a sphere

texts:
I have created a 3D rendering of the moon, which is represented as a sphere with a radius of N pixels.

I also have selenographic coordinates.  For example, the coordinates of the Apollo 11 landing site are 0.67408, 23.47297.  I'd like to show this coordinate on my sphere.

Here's some possibly relevant code, which is used for conversion with Earth coordinates:

function latLongToVector3(lat, lon, radius, height) {
    var phi = (lat)*Math.PI/180;
    var theta = (lon-180)*Math.PI/180;

    var x = -(radius+height) * Math.cos(phi) * Math.cos(theta);
    var y = (radius+height) * Math.sin(phi);
    var z = (radius+height) * Math.cos(phi) * Math.sin(theta);

    return new THREE.Vector3(x,y,z);
}

Is this the correct way to map selenographic coordinates onto a sphere?


texts: Are there ever any simultaneous transits of both Mercury and Venus as seen from the Earth?

texts:
Transits of Mercury happen fairly frequently due to its short period, but transits of Venus are less frequent.  I've looked over the data available to me and found that there have been transits of both planets in the same calendar-year in 1631 (Nov & Dec) and in 1769 (Jun & Nov).

What would really be interesting is if the two planets transited the Sun at the same time! With the rule being, that the second transit begins before the first transit completes.

Can we figure out if they have ever so transited, or if they ever will so transit?


texts: How big would a disc have to be to blot out the sun covering 5% of the land mass of earth?

texts:
Like this, but floating in space.

I imagine that this would also be influenced by the distance of the object from the sun, and that there is some ideal distance for this object to be? (and obviously the distance of the Earth from the Sun as well)


texts: does a minority of all planetary systems show transits?

texts:
do most or only a minority of all planetary systems to show transits?

I feel like its minority due to the random orientation of their orbits?

But the thing is I seem to find conflicting information.

Could someone explain which answer is correct because now I am confused?


texts:
In wikipedia's article on the phases of Venus it says:

The Ptolemaic system would never allow Venus to be fully lit from the perspective of the Earth, as this would require it to be on the far side of the Sun, which is impossible if Venus's orbit in its entirety is between the Earth and the Sun,

But the moon's orbit "in its entirety is between the Earth and Sun", and is "fully lit from the perspective of the Earth" once a month, so how can we conclude that Venus revolves around the Sun?


texts: Which class of objects in Solar System carries more mass?

texts:
Could you help me sorting this list in terms of total mass carried by these classes of objects?

The star (Sun) - it's very big but only one.
The planets (incl. dwarf planets). Certainly beaten by the Sun, but...
the asteroids, meteorites, comets, all that rubble. They aren't heavy individually but there's a lot of them.
the interplanetary medium. It's very low density, but it overshadows the rest by far in terms of volume occupied.

I'm not sure where to attach the moons - some of them are larger than some dwarf planets, some are definitely more asteroid-sized. I guess - whoever decides to answer this, is free to pick where they'd like to include the moons (or maybe to split them between the two classes, say, attaching the ones with hydrostatic equilibrium to dwarf planets, and bundling the rest with the rubble)?


texts: Are we less safe now that Arecibo is no more?

texts:
update 3: uhoh!

Source: Reddit
update 2: Sadly, parts of the dish have collapsed, several cables have snapped, and the Arecibo radio telescope is now decommissioned and demolition has been recommended because it is now too unsafe to do anything else.

NSF begins planning for decommissioning of Arecibo Observatory’s 305-meter telescope due to safety concerns
Iconic radio telescope in Puerto Rico to be demolished

related in Space SE: How many total hours had Arecibo transmitted a signal?

update 1: Science November 9, 2020: Second cable breaks at Puerto Rico’s Arecibo telescope

The NASA News item Arecibo Observatory Returns from Tropical Storm Isaias Lockdown to Track Asteroid for NASA begins:

The National Science Foundation facility, managed by the University of Central Florida, determines that a newly discovered asteroid won’t threaten Earth

Of course right after this there was a serious failure, a support cable for the focal plane instrumentation broke and seriously damaged the dish as well.
Are we currently at elevated risk to getting hit by an asteroid now that Arecibo can not help in making precise range-rate measurements of asteroids? Optical tracking provides detection and some orbit reconstruction, but the precise range and radial velocity measurements that can be done by radar can provide very precise data that can help distinguish an impact trajectory from one that will miss.
Could the Five-hundred-meter Aperture Spherical Telescope fill in for Arecibo's role in asteroid astrometry?


texts: What are angular sizes of thin and thick disks in our Galaxy?

texts: What are diapasons of galactic coordinates for thin and thick disks of our Galaxy?


texts: Why does Neptune only have one major moon?

texts:
Neptune is profoundly anomalous when it comes to moons among the gas giants.  Jupiter has its four massive moons, Saturn has a several large spherical bodies and Uranus, a world similar in size to Neptune has five or so major moons.

Yet Neptune, the ice giant which sports one of the largest hill sphere radii in the solar system only has one major moon, of which is likely to be ripped to shreds in the future.

I find it peculiar because Uranus has five major moons yet Neptune only has one major moon.  What explains this disparity?  Did some disruption happen to earlier moons Neptune may have had?  What's known about this?


texts:
If I understand correctly, the H-alpha and Ca II K lines are absorption lines of the sun and allow to see the chromosphere. Following the Kirchhoff-Bunsen law, an absorption line is produced by a gas which is cooler than the source of light.

However the chromosphere is hotter than the photosphere and the temperature increases outward (https://www.researchgate.net/figure/The-figure-plots-profiles-of-mass-density-dashed-line-and-temperature-solid-line-in_fig6_312376685). This is in contradiction with the previous hypothesis. Then how can we see the chromosphere in absorption line? What am I missing in the reasoning? Is the Krichhoff-Bunsen law only valid for a thin medium and here we would be in presence of a thick one?

thank you in advance for your help.


texts: Was the Universe expanding before the beginning of inflation?

texts: My point of view is that first there was the Big Bang singularity, and then the period of inflation which resulted in the observable Universe becoming many times bigger. But was the Universe expanding between the Big Bang singularity and the beginning of inflation?


texts: Sound can't travel through outer space. But if it could, how loud would the Sun be? Would the sound be dangerous to life on Earth, or would we barely hear it from this distance?


texts: Can expansion of space cause the motion of galaxies, does expansion of space do effect motion of our sun around center and earth around Sun?

texts: I'm just wondering that expansion is anyway related to in effecting motion of a star under the galaxies ? Planets are currently revolving around sun in helix motion, if galaxies has motion as well reason to expansion of space then it is possible that, that can effect anyway on that planetary motion around stars ?


texts: Jupiter with a mobile phone and Celestron Astro FI 102mm Maksutov

texts:
My name is John, I am new here and now that I am retired I decided to get myself my first ever telescope, why I never got one before I have no idea...probably too busy.
So I thought I would try and get some pics last night from a cloudy  Ross on Wye, Herefordshire. England. UK. Using my mobile as mentioned in the manual.
I appreciate they are not like the amazing ones here but it is my first try. I take it the large black disc is probably a reflection of the mirror? If not can you please tell me what it is. Why does this happen?
I would like to get a different eyepiece other than the 10mm and 25mm I would like a zoom can anyone suggest a reasonable one?
Anyway here are my attempts. Thanks


texts:
I have this problem: I want to calculate the declination of the star in the year 5000. And I know RA (right ascension) and Dec (declination) of the star right now.

How can I calculate it using spherical trigonometry.

  I know that the declination changes over time due to the precession
  and somewhere I read that I should use spherical triangle, but I am not
  really sure how to use it.

Thank you for your ideas.


texts: What is the difference between solar wind and solar radiation?

texts: Is there a difference between solar wind and solar radiation or are they just different ways of describing the same thing?


texts: Confusing satellite-like object seen in night sky

texts:
I was staring up at the stars around 11:30 PM and noticed an object moving. It had no lights, and was not bright whatsoever. It was moving smoothly and in a straight line, except extremely fast, and dead silent. It was fairly large too, compared to how big stars look. Never would have noticed it had I not been looking at that particular region of the sky at the exact moment I did. It had to have crossed the sky in less than a minute. In fact, I feel like I've seen this same kind of thing once or twice before.

My question is what did I see? The fact that it was extremely dim, extremely fast, and produced no sound really confuse me.


texts:
I've been reading about satellite observations (such as this one).
The writer uses terms such as these to define the orbit:

1 41584U 16036A   16167.96105997 0.00000000  00000-0  00000+0 0    07
2 41584   7.5055 353.7008 0046333  41.2140 319.1375  1.00195548    05

rms 0.004 deg      from 9 obs June 14.70 - June 16.79  (2.09 day arc)

What does each variable refer to, and what's the easiest way to view these on a map (either online or in an app of some kind?)


texts: How can I measure the mass of eclipsing binary stars?

texts: Is there any method to measure the mass of eclipsing binaries without using Doppler shift effect?


texts: If I know apparent planetary positions how do I calculate the date?

texts:
I have planetary position in retrograde  how do I calculate the date?
For eg, I know Mars was in retrograde in Scorpion (Sidereal 210 to 240 deg), how do I calculate the date and year of this?
As of now the only other info I have is that just days before this was a lunar eclipse where Sun was in Libra.
Just a year or two before this
Mars was in retrograde in first half of Leo and at this time Jupiter was placed at Capricorn. These are all sidereal observations.
P.S: This is regarding an ancient scripture citation where it was said Mars was in retrograde in Scorpion, current proposed dates range from 500 BC to 6000 BC. This is all the information I have, that Mars was is in retrograde in sidereal scorpion and I want to find possible date of this happening. The iterative approach would take too long, because the span is about thousands of years


texts: Newtonian reflector collimation question

texts:
After an initial collimation, I took my scope (150mm/750mm) out for a star test. While I could see concentric circles when the bright star is at the centre of the video, when the star is on the side of the view, I get an image typical of an uncollimated scope. Please refer the images below. The left image is how it appears when the bright star is at the centre. The right image is how it appears when the star is on the upper portion of the view.

Does my scope require further collimation, or is it fine the way it is now?


texts: Why does spectral class and U-V colour correlate slightly differently for main sequence, giant and supergiant stars?

texts:
In this diagram the $x$-axis is spectral type for stars and the $y$-axis is B-V colour.

Taking Series 1 as the Main Sequence, Series 2 as Giants and Series 3 as Super-giants. Why the spectral class for all these stars differ from the other ones?

If we were to say differences, for instance, how would one express that information?


texts: Why celestial objects appear brighter in the night sky than in photographs from closer proximity?

texts:
The moon, for example, appears very bright in the night sky, but quite dark from photographs on/near its surface.

Similarly, Mars and Venus appear as bright dots in the sky but in closer photographs they're quite dark.


texts: How does Hawking Radiation work exactly?

texts: I know that a particle and anti particle, virtual particles, spawn at the edge of an event horizon, and one particle falls into the black hole, and the other goes out, but how does the other know how to take energy from the black hole? I mean how does it? Does it just know to take energy from the black hole or does the antiparticle fall in, and the blackhole loses mass?


texts:
In 2018 the IAU produced a definition of an exoplanet:

The current official wokring definition of an exoplanet as amended by IAU Comission F2: Exoplanets and the Solar System in August 2018, reads as follows:

Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solare metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability ($M/M_{\rm central} < 2/(25+ \sqrt{621})\approx 1/25 $) are "planets" (no matter how they formed).
The minimal mass/ size required for an extrasolar object to be considered should be the same as that used in our Solar System.
We can expect this definition to evolve as our knowledge improves.

What is the L4/L5 instability and why was it chosen as part of the exoplanet definition?


texts: What is meant by sun's path through zodiac

texts:
I read somewhere that sun's daily course and complete cycle is visible in the ecliptic belt by zodiac constellations.

I didn't understand what that means closest I could find online is: https://www.youtube.com/watch?v=x1DqwC7lRpk

Is the sun's route visible during the night in some areas? Can someone explain?


texts: I think I didn't understand something about virial theorem for an $N$-body system, for instance the behaviour of virial ratio $T/\Omega$, with $T$ kinetic energy and $\Omega$ gravitational potential energy. As far as I understood, if the system is stationary, than virial ratio is $0.5$. This means on the other hand that the system is not expanding nor collapsing. But the behaviour of virial ratio is something like the plot in this question. Here the system starts from $T/\Omega < 0.5$, so it starts  collapsing and than reach a maximum. I would expect this maximum to be at $T/\Omega \sim 0.5$, since once the collapse stops the system is not accelerating nor decelerating, but it's about $0.75$. If the system is stationary at $0.5$ how can that maximum (and the next minimum too) be explained? I don't understand how virial ratio has a single value since it actually oscillates until converging to a specific value.


texts: Is the conversion from proto-star to main sequence an event or a process?

texts: When a proto-star becomes a main sequence star, is that something that happens in an instant when a certain threshold is met, or is it a process that takes a few thousand/million years?


texts: This effect is frequently discussed in the context of solar time series. In particular, it a relationship between soft x-ray (thermal emission) and hard x-ray (nonthermal emission) from the Sun.


texts: Can a gas giant have an other gas planet as satellite?

texts: Is it possible for a gas giant (A very big one) to have an other smaller gas planet as satellite?


texts: Why are so many sun-like stars found in multiple systems? How do planets form in such a situation?

texts:
It has been reported that perhaps half of nearby sun-like stars are found in multiple systems.

Why is this so common, and how do planets form in such a situation?

Kaitlin M. Kratter, “Sibling Rivalry Begins at Birth,” Nature, Vol. 518, 12 February 2015, p. 173


texts: Within a given image of a multiple-image producing gravitational lens, does Fermat's principle apply?

texts:
The question Does gravitational lensing provide time evolution information? is excellent!

When we see multiple images of the same object because of the phenomenon of gravitational lensing, do all the images show the lensed object at the same point in time?
Does it take the same amount of time for light from the lensed object to reach us no matter what image we look at or are we watching the lensed object at different points in time and if so, how big can the difference in time typically be?

The answer is generally no, different "images" are associated with substantially different time delays.
But the lens and mirror-based optical systems we used to see those phenomenon pretty much strictly obey Fermat's principle where every ray entering the telescope's aperture reaching given point on a detector takes the same amount of time through the telescope.
Of course gravitational lenses are not manufactured imaging systems, but usually wherever you see that an image is produced in nature, the thing producing that image will obey Fermat's principle.
Question: So at least within one given image of a multiple-impage producing gravitational lens, does Fermat's principle apply at least approximately?


texts: Why does iron consume more energy in the fusion process than it produces?

texts:
I understand that once a star starts fusing iron, it's doomed to collapse because iron fusion requires more energy than it releases in the process, allowing the opposing gravity of the star to cause it to collapse.

But why?  What makes iron special in this way?  It seems to occupy a pretty inconsequential spot in the middle of the period table among the transition metals.  So, why is it that iron breaks the rule in play for all the elements before it when it comes to fusion?

I noticed that on another question someone said that it wasn't iron, but nickel that was the first element that required more energy to fuse than it released- but every documentary and book I've read claims that it's iron.  So, if you're answer is "iron isn't the first element that requires more energy..." please explain why every other source I've ever heard is wrong!